Alkaline digestion of glass

Abstract

A method for the alkaline digestion of soda-lime glass comprising forming a mixture of soda lime glass and a hydroxide solution, the mixture having at least 100 grams of glass per litre of H2O, the hydroxide solution having a concentration of 1M or greater to thereby form an aqueous sodium silicate fraction having a silicate concentration of 50 g/L or greater (calculated as SiO2 equivalent) and a ratio of SiO2:M2O of at least 1, wherein M2O is an alkaline metal oxide, by digesting the glass in the mixture; and separating the aqueous sodium silicate fraction from solids. The solids contain calcium silicate hydrate and undissolved glass. The calcium silicate hydrate can be CSH treated with an acid to thereby dissolve soluble metals from the CSH and separating a liquid phase from a solid phase, the solid phase comprising SiO2 or silica gel.

The invention claimed is: 
     
       1. A method for the alkaline digestion of soda-lime glass, the method comprising:
 a) forming a mixture of soda lime glass and a hydroxide solution, the mixture having at least 100 grams of glass per litre of H 2 O, the hydroxide solution having a concentration of 1M or greater to thereby form, by digesting the glass in the mixture at atmospheric pressure:
 an aqueous sodium silicate fraction having a silicate concentration of 50 g/L or greater (calculated as SiO 2  equivalent) and a ratio of SiO 2 :M 2 O of at least 1, wherein M 2 O is an alkaline metal oxide; and 
 a solid material comprising a calcium-silicate-hydrate; 
 
 b) separating the aqueous sodium silicate fraction from the solid material comprising the calcium-silica-hydrate; 
 c) treating the solid material comprising the calcium-silicate-hydrate with an acid to dissolve soluble metals from the calcium-silicate-hydrate, thereby producing a liquid phase and a solid phase, wherein the solid phase comprises SiO 2 ; 
 d) separating the solid phase comprising SiO 2  in step (c) from the liquid phase; and 
 e) returning at least some of the separated solid phase comprising SiO 2  to step (a). 
 
     
     
       2. A method as claimed in  claim 1  wherein the mixture formed in step (a) comprises from 100 to 1500 g glass per litre H 2 O. 
     
     
       3. A method as claimed in  claim 1  wherein the soda lime glass that is present in the mixture in step (a) is in particulate form, wherein the soda lime glass is provided in the form of glass particles having a size distribution with a maximum particle size of 50 mm. 
     
     
       4. A method as claimed in  claim 1  wherein the soda lime glass comprises waste glass or recycled glass. 
     
     
       5. A method as claimed in  claim 1  wherein the hydroxide solution comprises an alkali metal hydroxide solution. 
     
     
       6. A method as claimed in  claim 5  wherein the alkaline metal hydroxide solution has a hydroxide concentration of from 1M to 10M. 
     
     
       7. A method as claimed in  claim 1  wherein the mixture of glass and alkali metal hydroxide formed in step (a) results in the digestion of the glass, and residence time in step (a) is greater than 1 hour. 
     
     
       8. A method as claimed in  claim 1  wherein step (a) is conducted at a temperature of from 50° C. to the boiling point of the mixture. 
     
     
       9. A method as claimed in  claim 1  wherein the mixture in step (a) is controlled such that the addition of water, soda-lime glass and alkali metal hydroxide occurs at a stoichiometric ratio that is consistent with a desired SiO 2 :M 2 O ratio in the sodium silicate solution. 
     
     
       10. A method as claimed in  claim 9  wherein the molar ratio of SiO 2 :Na 2 O, considering both glass and alkali metal hydroxide, is between 1:1 and 4:1 with the total molar concentration of SiO 2 +Na 2 O being >5M. 
     
     
       11. A method as claimed in  claim 1  wherein the step of separating the aqueous sodium silicate from solids takes place at elevated temperatures. 
     
     
       12. A method as claimed in  claim 1  wherein the step of separating the aqueous sodium silicate from the solid material in step (b) further comprises adding additional water to enhance separation. 
     
     
       13. A process as claimed in  claim 1  wherein the solid phase in step (d) comprises SiO 2  gel. 
     
     
       14. A process as claimed in  claim 1  wherein in step (d) the liquid phase separated from the solid comprises a leachate containing dissolved metals and the leachate is treated to precipitate metal compounds and optionally to separate the metal compounds from the liquid phase following separation of the leachate from the solid phase. 
     
     
       15. A process as claimed in  claim 13  wherein purity of the SiO 2  gel is improved by leaching iron salts from the SiO 2  gel by using acid to decrease pH to <3 and subsequently washing the solid SiO 2  in a dilute acid solution. 
     
     
       16. A method as claimed in  claim 1  wherein in step (e) the SiO 2  also contains undigested glass and some or all of the SiO 2  and undigested glass is returned to step (a). 
     
     
       17. A method as claimed in  claim 1  wherein the soda lime glass that is present in the mixture in step (a) is in the form of particles at least some of which have a size of at least 384 μm. 
     
     
       18. A method as claimed in  claim 1  wherein the soda lime glass that is present in the mixture in step (a) is in the form of particles wherein the particle size distribution has a d(0.9) of at least 384 μm.